Adiabat Shaping of Direct-Drive OMEGA Capsules Using Ramped Pressure Profiles

Transcription

1 Adiabat Shaping of Direct-Drive OMEGA Capsules Using Ramped Pressure Profiles a r Lagrangian coordinate K. Anderson University of Rochester Laboratory for Laser Energetics 44th Annual Meeting of the American Physical Society Division of Plasma Physics Orlando, FL November 2002

2 Collaborators Laboratory for Laser Energetics, University of Rochester R. Betti T. J.B. Collins Lawrence Livermore National Laboratory M. M. Marinak S.W. Haan

3 Motivation A shaped adiabat combines 1. High capsule adiabat at the ablation front, resulting in high ablative stabilization of the Raleigh Taylor (RT) instability: 1 g RT ª 0.94 kg 2.7 kv abl, V abl ~ a 3/5 2. Low fuel adiabat, giving high compression and high yield Shaping with a ramped pressure profile lends flexibility to adiabat shaping TC R. Betti et al., Phys. Plasmas 5, 5 (1998).

4 Outline Explanation of adiabat-shaping physics using ramped pressure profiles Analysis of shock-timing constraints Comparison of adiabat shapes from various prepulses TC6028

5 Shaping the adiabat with pressure relaxation requires a weak prepulse followed by a power shutoff and the main laser pulse Laser power Main Pulse Prepulse Shutoff Time The main pulse sets the adiabat by launching a strong shock through a relaxed pressure profile. TC6029

6 The shock from the main pulse shapes the shell adiabat as it travels up the relaxed density and pressure profile Decaying shock induced by prepulse Main shock Pressure Adiabat r r Shock timing must be within ~50 to 100 ps to avoid yield degradations of the order of 25%. TC6030 r r

7 If the main pulse starts too soon, the shocks meet inside the shell, setting the rear surface on a high adiabat Shocks meet at back Shocks meet inside the shell Density a r a r 10 5 Adiabat 0 Lagrangian coordinate 0 0 Lagrangian coordinate 0 Outer Inner Outer Inner TC6031

8 If the main pulse is delayed too much with respect to the prepulse, the back of the shell is set on a high adiabat Time Main shock Rarefaction Adiabat Main shock Adiabat Density Pressure Late main pulse Æ The inner shell surface is set on a high adiabat. TC6032

9 Two constraints on the main pulse: (1) the main and prepulse shocks must meet at the shell s rear surface and (2) the rear-surface adiabat is an assigned design parameter P foot Main Pulse Prepulse DT off TC6033 Time For a given prepulse (P prepulse, DT prepulse ) and rear adiabat, the laser shutoff time DT off and main-pulse foot power P foot are determined by these two constraints.

10 Dimensional analysis provides simple approximate formulas for the shutoff time and foot power DT off DT prepulse ª const m * m v Ê ˆ Ê P ˆ back Á ËM, Á shell ËP prepulse P foot P prepulse ª const m * n j Ê ˆ Ê P ˆ back Á ËM Á shell ËP prepulse DT off ( ns) ª 1.6 ( ) D shell ( mm) 10 r 0 g/cm È ù È ù È ù È ù Í ú Í ú Í ú Í ú a 0.17 DT prep ( ns) 90 P prep ( Mb) 0.25 back ( ) ( Mb) P foot ª 9 DT prep( ns ) 90 P prep r 0 g/cm È ù È ù È ù È ù ( Mb) Í ú Í ú Í ú Í ú a D shell ( mm) back TC6034

11 Cryogenic DT OMEGA capsule design and sample laser pulse shape Typical 1-D yields ~ Pulse shape: 250 ps, 1.2 TW prepulse 430 mm DT ice DT gas 339 mm 91 mm Laser power (TW) DT off Time (ns) DT off varies with prepulse pressure and duration and with capsule parameters. TC6035

12 Varying the intensity of the prepulse changes the steepness of the adiabat profile Results of varying the intensity of 50-ps square prepulses Adiabat TW 2.4 TW 4.8 TW 9.6 TW Lower-intensity prepulses give stronger shaping, shorter entropy-gradient scale length Ls. Convective instabilities are driven by entropy gradients with growth rates: 4 2 mm g conv ~ 1 L s 0 Distance TC6036

13 A study of 1.2-TW square prepulses shows that varying the length of the prepulse also affects the steepness of the adiabat profile Adiabat ps 150 ps 250 ps Shorter prepulses give stronger shaping and shorter entropy-gradient scale length L s. 4 2 mm 0 Distance TC6037

14 Conclusion Adiabat shaping combines high fuel compression with high ablative Rayleigh Taylor stabilization. Adiabat shaping can be done using a short prepulse followed by a period of laser shutoff and the main drive pulse. 1-D simulations have verified adiabat shaping for various prepulses. This method of adiabat shaping allows flexibility in setting adiabat shapes. Ongoing research: 2-D simulations to evaluate capsule stability for various adiabat shapes. Effects of both Rayleigh Taylor stabilization and convective instability will be studied. TC6112